Speech and data signals are more and more being transmitted optically rather than electrically, however switching is performed in the electrical mode. Optical switching has been suggested particularly in order to obviate the need for opto-electronic interfaces which add to the system complexity and cost. However, the relatively low operating speed of currently available optical switch elements places a severe restriction on the system bit rate. The present invention is concerned with a switching architecture that can be embodied in both optical and electronic form
For the circuit switching of digital channels TST (Time-Space-Time) or STS (Space-Time-Space), for example, are used in 64 kbit/s networks. These types of switching can be made to be non-blocking or to have a low blocking probability but are characterised by:
(1) fixed length data, i.e. 8 bits, PA1 (2) fixed bandwidth per channel, PA1 (3) no contention for output channels.
Clearly with these basic structures there are a number of problems with handling packets.
In a multi-service SDH (Synchronous Digital Hierarchy) there can be a mixture of 8 bit information in Synchronous Transfer Mode (STM) and packets in Asynchronous Transfer Mode (ATM). With packets there is contention for channels because packets for a given output channel can arrive simultaneously on a number of input channels and temporarily overload that output channel. Thus internal queues are required and if these queues overload then traffic can be lost. With all known designs there is a finite probability that this will happen.
In an SDH multiplex, ATM cells and their headers are, once the frame overhead is removed, contiguous bits in the bit stream, whereas STM information at, say the 2 Mbit/s level is distributed throughout the frame. This means that incoming ATM information can be switched on the fly but a frame store is necessary before STM information can be switched.
In the past these conflicting requirements have resulted in switch designs that either deal with ATM switching in isolation or, if not, the circuit packet switching are segregated into separate modules. For example, in the RACE context (R&D in Advanced Communication Technologies in Europe), the RACE BLNT design ATM only (A. L. Fox et al "RACE BLNT: a technology solution for broadband networks." Integrated Broadband Services and Networks, IEE Conference Publication No. 329, October 1990, pp 47-57), the ATMOSPHERIC switch had separate ATM and STM sections (D. G. Fisher et al "An open/network architecture for integrate broadband communications". Integrated Broadband Services and Networks, lEE Conference Publication No. 329, October 1990, pp 73-78). The BERKOM project uses an ATM switch (H. Armbruster et al "Phasing-in the universal broadband ISDN: initial trials for examining ATM applications and ATM systems" Integrated broadband Services and Networks. lEE Publication No. 329, October 1990, pp 200-205). Gauss is an ATM switch (R. J. F. de Vries "Guass: a single-stage ATM switch with output buffering". Integrated Broadband Services and Networks, lEE Conference Publication No. 329, October 1990, pp 248-252. The ATMOSPHERIC switch also contains an overload policing function to prevent users trying to use more resources than were negotiated at call set-up time. Another known switch, which is a packet only switch is Knockout (Y-S Yen et al "The Knockout Switch: a simple, modular architecture for high-performance packet switching" lEE Journal on selected Areas in Communication, Vol SAC-5, No. 8, October 1987, pp 1274-1283). The Knockout switch uses a fully interconnected switch fabric topology (i.e. each input has a direct path to every output) so that no switch blocking occurs where packets destined for one input interfere with (block or delay) packets going to different outputs. It is only at each output of the switch that one encounters the unavoidable congestion caused by multiple packets simultaneously arriving on different inputs all destined for the same output. Taking advantage of the inevitability of lost packets in a packet-switching network, the Knockout switch uses a concentrator design at each output to reduce the number of separate buffers needed to receive simultaneously arriving packets. Following the concentrator, a shared buffer architecture provides complete sharing of all buffer memory at each output and ensures that all packets are placed on the output line on a first-in first-out basis. Knockout appears to be the first switch design that used a broadcast approach, i.e. all the incoming channels broadcast their outputs to all the outgoing channels. Gauss also uses a broadcast approach which gives it its non-blocking property. Gauss is specific to the RACE environment and is modular at the STM-1 level. It differs principally from Knockout in the way concentration and output data queuing is achieved. A further switch construction is disclosed in U.S. Pat. No. 4,740,953 which describes a time division speech path switch having a plurality of speech path memories for each highway and wherein input highway information is stored simultaneously in all of the speech path memories.